The present invention relates to a surface-treated copper foil which has been subjected to anti-corrosion treatment; to a method of producing the surface-treated copper foil; and to a copper-clad laminate employing the surface-treated copper foil.
Conventionally, surface-treated copper foil has been employed as a material for producing printed wiring boards, which are widely used in the electric and electronics industries. In general, surface-treated copper foil is bonded, through hot-pressing, onto an electrically insulating polymer material substrate such as a glass-epoxy substrate, a phenolic polymer substrate, or polyimide, to thereby form a copper-clad laminate, and the thus-prepared laminate is used for producing printed wiring boards.
As disclosed in Japanese Patent Application Laid-Open (kokai) No. Hei 04-318997, copper foil in which a zinc-copper-tin ternary alloy plating layer and a chromate layer, serving as anti-corrosion layers, are formed on a surface of the foil has been widely employed and exhibits excellent heat resistance characteristics (generally called UL heat resistance) and resistance to chemicals (particularly to hydrochloric acid) during the manufacture of printed wiring boards. Among these characteristics, the resistance to hydrochloric acid can be evaluated through the following procedure. In practice, a printed wiring board having a pattern obtained from copper foil is immersed for a predetermined time in a hydrochloric acid solution of predetermined concentration. Instead of measuring the amount of the hydrochloric acid solution which has penetrated into an interface between the copper foil pattern and the substrate of the wiring board, the peel strength before immersion and after immersion are measured. The percent loss in peel strength with respect to the initial peel strength is calculated, and the value is employed as an index of resistance to hydrochloric acid.
In general, as the line width of a copper pattern in a printed wiring board decreases, the copper foil for producing a printed wiring board requires higher resistance to hydrochloric acid. When the copper foil shows a large decrease in peel strength with respect to the initial peel strength, the interface between the copper foil pattern and the substrate readily absorbs a hydrochloric acid solution and the interface readily undergoes corrosion. In a printed wiring board produced from such copper foil, the copper circuit pattern is likely to drop out of the substrate, since the copper foil is treated with a variety of acidic solutions during fabrication of printed wiring boards.
In recent years, the thickness, weight, and dimensions of electronic and electric apparatus have been steadily decreasing, and therefore, there is corresponding demand for further reduction in the width of the copper pattern line to be formed on printed wiring boards. In this connection, there is additional demand for copper foil having higher resistance to hydrochloric acid to be used in the production of printed wiring boards.
Actually, the present inventors have produced the copper foils, on a trial basis, by means of the methods disclosed in the above-mentioned publications, and have performed a test for evaluating resistance to hydrochloric acid by use of copper pattern specimens having a line width of 1 mm and obtained from the copper foils. Results similar to those disclosed in those publications have been attained. At the time of filing the aforementioned patent applications, resistance to hydrochloric acid was generally evaluated by measurement on a copper pattern specimen having a line width of 1 mm. Although no mention is made in the specifications of these publications, the test is thought to be carried out by measurement on a copper pattern specimen having a line width of 1 mm and obtained from the copper foils. Some references; i.e., kokai patent publications specified below, disclose methods for enhancing resistance of copper foil to hydrochloric acid, including subjecting a surface to be bonded to a substrate to treatment with a silane coupling gent.
However, through formation of a copper pattern having a line width of 0.2 mm with copper foil prepared for test purposes according to the methods disclosed in the above kokai patent publications and testing for evaluation of resistance to hydrochloric acid, the inventors have found that most specimens exhibited a percent loss in peel strength in resistance against hydrochloric acid degradation of 15% or more. Concerning current tests to evaluate resistance of copper foil against hydrochloric acid, it has been accepted that product quality which meets the recent trend toward reduction of the line width of copper patterns cannot be guaranteed unless evaluation tests are performed for copper pattern specimens having a line width of approximately 0.2 mm. For example, even though copper foil attains a percent loss in peel strength in resistance against hydrochloric acid of approximately 3.0% as measured on a copper pattern prepared from the copper foil and having a line width of 1 mm, the same copper foil attains a percent loss in peel strength in hydrochloric acid degradation of more than 10% as measured on a copper pattern prepared from the copper foil and having a line width of 0.2 mm. In some cases, the percent loss in peel strength reaches 20% or more. Therefore, the quality of copper foil for producing fine-pitch copper patterns cannot be evaluated through a conventional test method including measurement on a copper pattern having a line width of 1 mm.
In a copper-clad laminate, the silane coupling agent is present between an anti-corrosion layer formed on copper foil and a substrate formed of any of a variety of organic materials. However, details of the silane coupling agent; e.g., the method of employment thereof, have not been sufficiently studied. Heretofore, there have been filed several patent applications with regard to copper foil employing a silane coupling agent.
For example, Japanese Patent Publication (kokoku) Nos. Sho 60-15654 and Hei 02-19994 disclose copper foil in which a zinc or zinc alloy layer is formed on a surface of the foil, a chromate layer is formed on the zinc or zinc alloy layer, and a silane coupling layer is formed on the chromate layer. Judging from consideration of the entirety of the aforementioned patent publications, these patents focus on drying treatment performed after formation of a chromate layer, and treatment with a silane coupling agent performed after drying. However, the present inventors have found that copper foil of expected performance cannot be obtained when a specific factor is not controlled; i.e., performance and quality of copper foil, particularly resistance to hydrochloric acid and moisture, vary greatly between lots even when the copper foil is produced, on a trial basis, by means of the disclosed methods.
Japanese Patent Publication (kokoku) No. Hei 02-17950 discloses that treatment of copper foil with a silane coupling agent is able to improve resistance to hydrochloric acid, but does not specifically disclose the moisture resistance of copper foil. In recent years, there have arisen problems which correspond to trends toward formation of miniature wiring and multilayer printed wiring boards and in the field of packaging of semiconductor devices. Due to the employment of a copper-clad laminate having poor moisture resistance, delamination of multilayer printed wiring boards and poor pressure-cooker performance of packaged semiconductor devices have occurred.
As described above, regarding formation of a silane coupling agent layer on an anti-corrosion layer comprising a ternary alloy layer on copper foil and a chromate layer formed on the ternary alloy layer, it is considered that no attained invention takes into consideration combination of the silane coupling agent and the anti-corrosion layer, surface conditions of the anti-corrosion layer during adsorption of the silane coupling agent, and drying conditions, and brings out the maximum effect in the employed silane coupling agent.